Optical devices using reflecting polarizing materials

ABSTRACT

Optical devices using reflective polarizers and, in particular, diffusely reflective polarizers are provided. Many of the optical devices utilize the diffusely reflecting and specularly transmitting properties of diffusely reflecting polarizers to enhance their optical characteristics. The optical devices include a lighting system which uses a reflector formed from a diffusely reflecting polarizer attached to a specular reflector. Another optical device is a display apparatus which uses a diffusely reflecting polarizer layer in combination with a turning lens which folds shallow angle light toward a light modulating layer. Other optical devices exploit the depolarizing characteristics of a diffusely reflecting polarizer when reflecting light. Still other optical devices use diffusely reflecting polarizers to recycle light and improve display illumination.

FIELD OF THE INVENTION

The present invention generally relates to optical devices usingreflective polarizers and, more particularly, to optical devices usingdiffusely reflecting polarizing materials.

BACKGROUND OF THE INVENTION

Reflecting polarizers generally include materials which transmit lightof a first polarization and which reflect light of a second, differentpolarization. Reflecting polarizers include, by way of example and notof limitation, diffusely reflecting polarizers, multilayer reflectivepolarizers, and cholesteric reflective polarizers. Examples of diffuselyreflecting polarizing materials includes those disclosed U.S. Pat. Nos.5,783,120 and 5,825,543 and in PCT Patent Application Publication Nos.WO 97/32223, WO 97/32224, WO 97/32225, WO 97/32226, WO 97/32227, and WO97/32230, the contents of all of which are incorporated herein byreference. Examples of multilayer reflective polarizers are described inU.S. Pat. No. 5,882,774, the contents of which are incorporated hereinby reference. Examples of cholesteric reflective polarizers aredescribed in EP 606 940 and U.S. Pat. No. 5,325,218, the contents ofboth of which are incorporated herein by reference.

SUMMARY OF THE INVENTION

The present invention provides optical devices using reflectingpolarizers, such as diffusely reflecting polarizers. In one embodiment,a display apparatus is provided. The display apparatus includes a lightmodulating layer having first surface and a light cavity for providinglight to the light modulating layer. A light guide is disposed toreceive light from the light cavity and output light at a low anglerelative to the first surface of the light modulating layer. A turninglens is disposed to receive the low angle light output from the lightguide and redirect light towards the light modulating layer. The displayapparatus further includes a diffusely reflecting polarizer disposedbetween the turning lens and the light modulating layer for receivingthe redirected light and transmitting a component of the redirectedlight having a first polarization toward the light modulating layer anddiffusely reflecting a component of the redirected light having a secondpolarization different than the first polarization.

A lighting system, according to an embodiment, includes a light sourcefor providing light and a reflector which includes a diffuselyreflecting polarizer disposed closer to the light source and a specularreflector attached to the diffusely reflecting polarizer and disposedfurther from the light source. In use, a component of the light having afirst polarization is transmitted by the diffusely reflecting polarizer,specularly reflected by the specular reflector, and specularlyretransmitted through the diffusely reflecting polarizer to providespecularly reflected light of the first polarization having a firstdistribution. A second component of the light which has a second,different polarization is diffusely reflected by the diffuselyreflecting polarizer (without reaching the specular reflector) toprovide diffusely reflected light having a second distribution differentfrom the first distribution. The diffused light may be used as generalambient light while the specularly reflected light of the firstpolarization may be used as task lighting, for example.

A display apparatus, in accordance with another embodiment, includes alight modulating layer and a light cavity adapted to provide light to alight modulating layer. The apparatus further includes a diffuselyreflecting polarizer, disposed between the light modulating layer andthe light cavity, for transmitting a component of the light provided bythe light cavity having a first polarization for viewing and diffuselyreflecting a component of the light received from the light cavityhaving a second polarization. The light cavity typically reflectsincident light, e.g., light diffusely reflected by the polarizer, with afirst degree of depolarization. The polarizer diffusely reflects thelight with a second degree of depolarization greater than the firstdegree of depolarization to provide light of the first depolarization.Due to the depolarization, at least a portion of the diffusely reflectedlight of the first polarization is reflected by the light cavity withoutpolarization toward the diffusely reflecting polarizer for transmissiontherethrough.

In another embodiment, a display apparatus is provided which includes alight cavity for providing light and a diffusely reflecting polarizerdisposed to receive the light. The diffusely reflecting polarizerdiffusely reflects light of a second polarization toward the lightcavity and transmits light of a first polarization. The diffuselyreflective polarizer has a selected dispersed phase concentration whichprovides a desired gain distribution.

A display apparatus, in another embodiment, includes a emissive elementfor providing light and a contrasting enhancing filter disposed on theviewing side of the emissive element. The contrast enhancing filterincludes an absorbing polarizer and a reflecting polarizer disposedcloser to the emissive element than the absorbing polarizer. Thecontrast enhancing filter may, for example, further include one or moretint layers above and/or below the diffusely reflecting polarizer. Thereflecting polarizer may, for example, be a diffusely reflectingpolarizer.

The above summary of the invention is not intended to describe eachillustrated embodiment or every implementation of the present invention.The figures and the detailed description which follow more particularlyexemplify these embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of thefollowing detailed description of various embodiments of the inventionin connection with the accompanying drawings, in which:

FIG. 1A illustrates an exemplary display apparatus in accordance with anembodiment of the invention;

FIG. 1B illustrates an exemplary display apparatus in accordance withanother embodiment of the invention;

FIG. 1C illustrates an exemplary display apparatus in accordance withyet another embodiment of the invention;

FIG. 2 illustrates an exemplary display apparatus in accordance withanother embodiment of the invention;

FIG. 3A illustrates an exemplary projection display system in accordancewith an embodiment of the invention;

FIG. 3B illustrates an exemplary projection display system in accordancewith another embodiment of the invention;

FIG. 3C illustrates an exemplary projection display system in accordancewith another embodiment of the invention;

FIG. 3D illustrates an exemplary microdisplay system in accordance withanother embodiment of the invention;

FIG. 3E illustrates an exemplary microdisplay system in accordance withyet another embodiment of the invention;

FIG. 3F illustrates an exemplary microdisplay system in accordance withyet another embodiment of the invention;

FIG. 3G illustrates an exemplary microdisplay system in accordance withstill another embodiment of the invention;

FIGS. 4A-4B illustrates an exemplary transflective display apparatus inaccordance with an embodiment of the invention;

FIG. 5A illustrates a display having a conventional contrast enhancingfilter;

FIG. 5B illustrates a display having a contrast enhancing filter inaccordance with an embodiment of the invention;

FIG. 5C is a graph illustrating relative brightness as a function oftint for contrast enhancing displays;

FIG. 5D is a graph illustrating contrast ratio as a function of tint forcontrast enhancing displays;

FIG. 5E is a graph illustrating another contrast characteristic graphfor contrast enhancing displays;

FIG. 6 illustrates an exemplary display apparatus in accordance with anembodiment of the invention;

FIG. 7A illustrates an exemplary lighting system in accordance with anembodiment of the invention;

FIG. 7B illustrates an exemplary reflecting material for use in thelighting system of FIG. 7A in accordance with an embodiment of theinvention;

FIG. 8A-8D illustrate exemplary security labels using diffuselyreflecting polarizers in accordance with embodiments of the invention;and

FIG. 9 illustrates an exemplary display apparatus in accordance withstill another embodiment of the invention.

While the invention is amenable to various modifications and alternativeforms, specifics thereof have been shown by way of example in thedrawings and will be described in detail. It should be understood,however, that the intention is not to limit the invention to theparticular embodiments described. On the contrary, the intention is tocover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

The present invention is believed to be applicable to a number ofdifferent optical devices using reflecting polarizers. Aspects of theinvention are particular suited to the use of diffusely reflectingpolarizers. While the present invention is not so limited, anappreciation of various aspects of the invention will be gained througha discussion of the examples provided below.

Diffusely reflecting polarizers (DRPs) typically specularly transmit acomponent of incident light of a first polarization and diffuselyreflect a component of the light having a second polarization. The termsspecular and diffuse are relative terms which can vary depending on thecontext of the application. As used herein, specularly transmitted lightgenerally refers to light transmitted without substantial deviation fromits incident path. The term diffusely reflected light generally refersto light reflected with considerable diffusion. By way of example andnot of limitation, specularly transmitted light may refer to lightdeviated from its incident path by about 8 degrees or less, whilediffusely reflected light may refer to light deviated by about 8 degreesor more from a reflection angle equal to an incident angle of the light.

Many of the examples illustrate display apparatus or other opticaldevices which include light sources, light guides and/or light cavities.The term light source will be used herein to refer to a source of light,such as a light bulb. The terms optical cavity and light cavity will beused interchangeably herein to refer to a cavity which provides light.Such a cavity may include one or more of the following elements: a lightsource, a light guide or other transparent medium, and one or morespecular or diffuse reflectors.

FIG. 1A illustrates a display apparatus using a diffusely reflectingpolarizer according to one embodiment. The exemplary display apparatus100 includes a light modulating system 130 and an optical cavity 120 forproviding light to the light modulating system 130 and illuminating thedisplay apparatus 100. In this embodiment, the light modulating system130 includes a first polarizer 105, a first transparent substrate 106, alight modulating layer 107, a second transparent substrate 108, and asecond polarizer 109. The example display apparatus 100 may, forexample, be a liquid crystal display (LCD) having liquid crystal lightmodulating layer 107. First polarizer 105 is typically a dichroicpolarizer which transmits light of a desired polarization and absorbslight of an undesired polarization. The optical cavity 120 typicallyincludes a light source 111 and a light guide 102 for receiving lightfrom the light source 111. The light source 111 may, for example, be alinear light source, such as a cold cathode fluorescent tube, or CCFT.The light guide 102 may, for example, be made of a transparent plasticmaterial such as polymethylmethacrylate (PMMA). Light guide 102 istypically wedge shaped, as shown in FIG. 1A, but other shapes may beused.

In operation, light from light source 111 is directed, with the aid ofreflector 112, into light guide 102. Most rays of light travelingthrough light guide 102 impinge upon surfaces 114 and 116 at anglesgreater than the critical angle, and are therefore totally reflected bytotal internal reflection (TIR). In order to extract light from lightguide 102, small surface irregularities or local angular changes may beincorporated into surface 114, to frustrate some of the total internalreflection, a phenomenon called frustrated total internal reflection(FTIR). Since guide 102 is typically relatively thin, light can onlytravel through it in a narrow range of directions. As a result, light(e.g., rays 151 and 153) leaving guide 102 tends to leave at relativelylow angles α relative to the surface 118 of the light modulating layer107. The angle α is typically less than 20 degrees in many applicationsand, in one embodiment, ranges from 10 to 20 degrees.

To redirect the light toward the light modulating layer 107, a turninglens 103 may be provided to fold the optical path of illuminating rays,such as rays 151 and 153, thereby directing light from the opticalcavity 120 to the light modulating layer 107. The example turning lens103 includes a structured surface 103 a facing the optical cavity 120and a relatively planar surface 103 b on the opposite side. Thestructured surface 103 a may, for example, include multiple prisms. Theturning lens 103 may, for example, bend the low angle rays to adirection substantially normal to the light modulating layer surface 118as shown. The optical cavity 120 may further include a reflector 101disposed adjacent to surface 116 to reflect light escaping from surface116 back into guide 102, where at least some of it will eventually,after one or more reflections, leave guide 102 through surface 114. Adiffuser 104 may, optionally, be included to, for example, widen therange of directions of viewability of display apparatus 100, since lightrays 151 and 153 may, with some light sources, be collimated into arelatively narrow range of directions. Optional diffuser 104 may also,in some cases, improve the appearance of display 100 in other ways, suchas providing a more uniform appearance to display 100.

The example display apparatus 100 further includes a diffuselyreflecting polarizer (DRP) disposed between the turning lens 103 and thelight modulating layer 107 for receiving the redirected light andtransmitting a component of the redirected light having a firstpolarization toward the light modulating layer 107 and diffuselyreflecting a component of the redirected light having a secondpolarization different than the first polarization toward the lightguide 102.

In one embodiment, a DRP layer 110 a is disposed between turning lens103 and diffuser 104, without being attached to other components (asshown on the left side of FIG. 1A). Alternatively, a DRP layer 110 b mayattached, e.g. laminated, to turning lens 103 (as shown on the rightside of FIG. 1A). In other embodiments, a DRP layer 110 c may beattached to a diffuser 104 (as shown in the left portion of FIG. 1B) ora DRP layer 110 d may be attached to both turning lens 103 and diffuserlayer 104 (as shown on the right side of FIG. 1B). Referring to FIG. 1C,a DRP layer 110 e may be integrated into the display apparatus 100 byattaching it to a diffuser 104, which is in turn attached to a polarizer105 (as shown on the left side of FIG. 1C). A DRP layer 110 f may beattached to turning lens 103 and to a diffuser 104, which is in turnattached to a polarizer 105 (as shown in the right side of FIG. 1C).Where the diffuser 104 is omitted, the DRP layer may, for example, beattached to the bottom side of the light modulating system (e.g. to apolarizer 105).

Furthermore, in the above embodiments, the diffuser may be omitted withthe DRP layer incorporating its diffusion functionality. For example,when a DRP layer is used in embodiments without a diffuser, the surfaceroughness of the DRP layer may be controlled to provide surfacediffusion. Alternatively, a DRP may be adjusted to provide bulkdiffusion by adjusting the degree of specular transmission of thetransmitted polarization state versus diffusion transmission of thetransmitted polarization state.

The diffusely reflecting polarizer 110 a-f and optical cavity 120 mayadvantageously be optimized for light recycling. In one embodiment, theoptical cavity 120 reflects light with little depolarization and thediffusely reflecting polarizer 110 a-f diffusely reflects light withsubstantial depolarization. In other words, the DRP layer 110 a-fdiffusely reflects light of the second polarization such that thereflected light includes a relatively large quantity of light having thefirst polarization. For example, the ratio of light of the firstpolarization to light of the second polarization may be 1:4 or more formany applications. In operation, the DRP layer 110 a-f transmits lightof a desired first polarization and diffusely reflects light of asecond, undesired polarization. Due to depolarization, the reflectedlight includes a relatively large component of the desired firstpolarization. The reflected light passes through turning lens 103 and isre-reflected with little depolarization by optical cavity 120.Re-reflected light of the first polarization subsequently transmitsthrough the DRP layer 110 a-f and re-reflected light of the secondpolarization is diffusely reflected and partially depolarized tocontinue the recycling process. As a result, more of the light producedby the light source 111 is utilized by the display apparatus 100. Inaddition, recycling of the light reflected by DRP layer 110 a-f over avariety of different optical paths tends to even out variations inillumination which sometimes occur in backlight illuminators.

While this embodiment works particularly well with reflectors providinglittle depolarization, the invention is not so limited. Other, moredepolarizing reflectors and optical cavities may be employed with orwithout substantially depolarizing DRP layers. In addition, in otherembodiments, the angular depolarization characteristics of the DRP andlight cavity may be set such that the optical cavity significantlydepolarizes light at incident angles containing relatively large amountsof non-depolarized light from the DRP and vice versa.

The diffusion of a DRP is typically much higher in the plane containingthe cross-stretch and normal axis compared to the plane containing thestretch and normal axis. Where the diffusion is desired to obscure theextraction pattern, the extraction pattern should be optimized for usewith the DRP. For example, if the DRP is oriented at 45°, the optimumorientation of, for example, the extraction pattern is typically at 0°.

In some embodiments, non-polarizing components may be included withinthe oriented film to achieve diffusion. For example, the precursor filmmay be coextruded or coated before stretching with a composition thatdoes not take on significant birefringence during stretching atconditions necessary to make the DRP. Examples of suitable compositionsinclude acrylic and styrene:acrylic copolymer macrospheres in a PMMAhost.

FIG. 2 illustrates a display apparatus having a DRP layer with aselected gain distribution. The example display apparatus 200 includes aDRP layer 220 and a light cavity 230 which, in the illustratedembodiment, utilizes a light source 232 and a reflector 233 to producediffuse light rays 231, incident upon the DRP layer 220. The displayapparatus 200 further includes a light modulating layer 210 which, inthe example embodiment, includes a liquid crystal layer 214 disposedbetween two polarizers 212 and 216. The DRP layer 220 transmits light221 having polarization P1, thereby separating it from light 223, whichis diffusely reflected back to cavity 230. Light 221(P1) is incidentupon first polarizer 212 of light modulating layer 210, which has itstransmission axis oriented parallel to the transmission axis of DRPlayer 220, so as to transmit light 225(P1′) having polarization P1′ intoliquid crystal layer 214. Polarization P1′ typically differs frompolarization P1 in that it is usually more purely linearly polarized, byvirtue of having passed through polarizer 212, which is typically anabsorbing dichroic polarizer capable of producing high levels of linearpolarization.

In the example embodiment, the liquid crystal layer 214 is made up of anarray of electronically addressable liquid crystal pixel elements whichare individually addressed by the application of electric fields throughan array of electrodes to align the liquid crystal material of eachpixel in either an optically inactive state (e.g., voltage-on, that isto say, when the electric field is applied) or in an optically activestate (e.g., voltage-off, or, when no electric field is applied). Asused herein, the term optically active means that the orientation of theplane of polarization of polarized light passing through the opticallyactive material is altered. The second polarizer 216 functions as ananalyzer to either transmit or block light transmitted by the pixelsmaking up liquid crystal layer 214, depending upon the direction ofpolarization of the transmitted light, as determined by whether eachpixel is in an optically active or optically inactive state. Byappropriately applying voltage to individual pixels in the liquidcrystal array in an imagewise manner, a viewable image is formed bydisplay 210. While a liquid crystal display with a liquid crystal layeris illustrated, the invention is not so limited. Other types of displaysmay benefit be employing selected gain distribution.

DRP layer 220 improves the light utilization of liquid crystal display210 by reflecting light 223 back to reflecting cavity 230, where aportion of it is re-reflected back toward DRP layer 220, which againtransmits that portion having polarization P1 and substantially reflectsthe remaining portion, thereby adding to the illumination of display210. This process, called light recycling, continues until all of light231 is either transmitted by DRP layer 220, with polarization P1, orlost to absorption. Light recycling has been found to significantlyincrease the light utilization. Increased light utilization can increasethe brightness of a liquid crystal display, for a given level of opticalcavity illumination, or, alternatively, it can allow a lower level ofoptical cavity illumination to achieve the same brightness, therebyreducing energy consumption. The DRP layer 220 and light cavity mayfurther be optimized to recycle light. For example, the DRP layer 220may, for example, substantially depolarize diffusely reflected light tofacilitate light recycling with a light cavity having lessdepolarization characteristics. Alternatively, the angulardepolarization characteristics of the two components may be set suchthat the light cavity significantly depolarizes light at incident anglescontaining relatively large amounts of non-depolarized light and viceversa, as noted above.

The example DRP layer 220 includes a selected gain distribution. Gaingenerally refers to the ratio of the luminance a display with a DRPlayer to the luminance of the display without the DRP layer. Theselected gain distribution can be provided by, for example, selectingthe concentration of the dispersed phase relative to the concentrationof the continuous phase. By way of example, increasing the concentrationof disperse phase relative to the concentration of continuous phasetypically increases the on-axis gain relative to the off-axis gain.Conversely, decreasing the concentration of disperse phase relative tothe concentration of continuous phase will typically decreases theon-axis gain relative to the off-axis gain. For example, with a filmproviding an on-axis gain of 1.35 and a 40 degree (relative to normal)gain of 1.29, by increasing its disperse phase concentration anddecreasing its continuous phase concentration, the film's on-axis gainmay increase to 1.44 and its off-axis 40 degree gain may decrease to1.18. Depending on the desired gain distribution, the concentration ofthe dispersed phase can be selected.

The table below illustrates on-axis gain and off-axis gain (−60 degreesto normal) of DRPs with different dispersed phase concentrations. TABLE1 Dispersed Phase On-axis Off-axis Concentration Gain Gain (−60°) 451.37 1.07 40 1.36 1.27 30 1.35 1.47

The selected gain distribution can be provided to improve theviewability of display 210 by controlling the light distribution seen byviewers such as 201 and 202, as typified by rays 211 and 213. A usefulparameter for describing luminance distribution is the half height angleθ, i.e., the angle at which the luminance is half the maximum luminance(which is assumed to be on-axis in this case). If, for example, ray 211represents the luminance of a bright portion of the image displayed bydisplay 210 when viewed at normal viewing angle, and ray 213 representsa ray emanating from the same point on the image having half of theluminance of ray 211, then angle θ, called the half height angle, is ameasure of the breadth of the light distribution for the displayedimage.

It will be appreciated that for a given level of lumination from rays221, increasing θ distributes the light available for viewing moreevenly, thereby lowering the maximum luminance, which typically occursat normal viewing angle, as represented by ray 211. In a display whichis to be viewed from a wide range of angles, this may be a desirablesituation. In situations wherein a single viewer is viewing the displayat substantially normal angles, however, it may be desirable to conserveenergy by reducing θ so as to brighten the display for that viewer.Typically, an increase in disperse phase concentration reduces angle θ,thereby narrowing the distribution of light available for viewing andproviding a brighter display for viewer 201, for example, while tradingoff brightness for viewer 202. Conversely, reducing the concentration ofthe disperse phase in layer 220 increases θ, thereby evening out thedistribution of light available for viewing the display.

A DRP layer having a particular gain distribution may be manufactured byselecting the relative concentrations of the dispersed phase andcontinuous phase to provide a desired gain distribution. This selectionmay take into account a concentration of a compatabilizer phase. It willbe appreciated that the ability to design the light distribution in thisway is a desirable feature for display designers, since it enables themto produce displays for a variety of different viewing applicationsmerely by selecting different DRP materials, based upon theconcentration of disperse phase present in layer 220.

FIG. 3A illustrates an exemplary projection display system in accordancewith another embodiment of the invention. The exemplary projectiondisplay system 300 utilizes an illuminator 310 to illuminate areflective imager 304, with the resulting image being reflected by apolarizing beam splitter 303, through projection lens 305, onto screen320. Illuminator 310 typically includes a light source 301, opticalenclosure 308, and beam conditioner 302. Beam conditioner 302 maycomprise lenses or other beam shaping components, optical filters toremove infrared or ultraviolet wavelengths of light, and reflectivepolarizing materials, such as a DRP layer. Optical enclosure 308 maycomprise an inner surface which is diffusely reflecting, or whichcontains polarization altering means such as birefringent layers.Optical enclosure 308 is not limited to a rectangular shape, but may beof any suitable shape, and may further contain structures on its innersurface which give it yet other effective internal optical shapes.

Polarizing beam splitter 303 typically comprises a reflective polarizingmaterial. It is desired that beam splitter 303 reflect image rays 353 ina substantially specular manner so as to preserve the image produced byimager 304. Therefore, it is advantageous that the disperse phase of anymultiphase reflecting material used in beam splitter 303 be in the formof uniformly oriented particles having a high, preferably substantiallyinfinite, radius of curvature, so as to reduce the diffuse component ofthe reflected light. Suitable particles could include flakes, platelets,or other particles having substantially flat, orientable surfaces.

In use, illuminator 310 illuminates imager 304 through polarizing beamsplitter 303. It is often desired that light 350 from illuminator 310 beprepolarized to, for example, polarization P1 by beam conditioner 302acting in cooperation with enclosure 308 to recycle light of undesiredpolarization. Beam splitter 303 further polarizes light 350 and providespolarized light 351 to the reflecting imager 304, which may, forexample, include an array of liquid crystal elements representing pixelsof a digital image. Depending upon the voltage applied to each pixel,the polarization of light 351 striking the pixel is either rotated orleft unrotated, and reflected back toward beam splitter 303. Beamsplitter 303 reflects light 353, which has been rotated in polarizationfrom polarization P1 to polarization P2, through projection lens 305,and then to screen 320, onto which the image created by reflectiveimager 304 is projected. Beam splitter 303 transmits light 352 havingunaltered polarization P1 back to illuminator 310, so that the pixelsrepresenting this polarization appear dark on screen 320. A portion ofthe light transmitted back to illuminator 310 may be reflected,repolarized, and recycled.

An alternative embodiment of the above system is shown in FIG. 3B,wherein the light reflected by beam splitter 303, rather than the lighttransmitted by beam splitter 303, is used as illumination for imager304. The imager 304 reflects light 301 of polarization P1 or P2depending on its pixel states. The beam splitter 303 then transmits P2polarized light 353(P2) and reflects P1 polarized light back toward theilluminator 310. In this case, beam splitter 303 may be a diffuselyreflecting polarizer, which transmits substantially specularly. Opticallayouts for projection displays of this type can be made more compactthan some conventional projection apparatus, since it is a feature ofthe diffusely reflecting polarizing materials disclosed in thereferences incorporated hereinabove that they can be made quiteinsensitive to angle of incidence, and therefore able to polarizestrongly diverging or converging beams, without the need for collimatingoptics.

Referring to FIG. 3C, an alternative projection system is portrayedwhich uses transmissive imager 306, rather than the reflective imagerused in the previous two systems. In this system, beam conditioner 302and, optionally, absorbing polarizer 307, provide polarized light 350,having polarization P1, which illuminates imager 306. Imager 306 mayagain include an array of liquid crystal elements representing pixels ofa digital image, which either transmits or blocks light 350 in animagewise manner to form the image which is then projected, throughprojection lens 305, onto screen 320. The illuminator 310 may furtherinclude a reflector for redirecting and randomizing light reflected bythe beam conditioner 302 toward the beam conditioner 302 fortransmission therethrough, thereby recycling light. In one embodiment,the beam conditioner 302 is a DRP. In one case, the DRP depolarizes thediffusely reflected light and/or the reflector may depolarize light sothat light not transmitted through the DRP can be recycled and latertransmitted therethrough.

Referring to FIG. 3D, an optical layout similar to that shown in FIG. 3Acan be used, with the exception that rather than projecting a real imageonto a screen, a magnified virtual image of imager 304 is seen by viewer1 using magnifying lens 305′. Displays which operate in this manner willhereinafter be called microdisplays. Referring to FIG. 3E, an opticallayout similar to that shown in FIG. 3B can be used, with the exceptionthat rather than projecting a real image onto a screen, a magnifiedvirtual image created by imager 304 is seen by viewer 1 using amagnifying lens 305′. This layout also has the advantage that the beamsplitter 303 may be a DRP, as the beam splitter 303 does not need to bespecularly reflective, but only specularly transmissive.

In an alternative embodiment similar to FIG. 3E, polarizing beamsplitter 303 is laminated or otherwise attached to transparent cube 306to form a more compact layout, as shown in FIG. 3F. In yet anotherembodiment, portrayed in FIG. 3G, light 350 from illuminator 310 enterslight guide 307, where it is polarized by diffusely reflecting polarizer303 while being reflected toward reflecting imager 304. Imager 304,which is typically made up of an array of liquid crystal elementsrepresenting pixels in a digital image, reflects polarized light 351with either unaltered polarization P1 or rotated polarization P2, asdetermined by the voltages applied to the pixel elements of imager 304.Light 353, having polarization P2, is the portion of the light reflectedfrom imager 304 which has the polarization transmissible by reflectivepolarizer 303, so that it passes through polarizer 303 to an optionalpolarizer 308. Polarizer 308 can be, for example, an absorbing polarizerwhich filters out any light having polarization P1, thereby preventingwrongly polarized light from reaching viewer 1. Viewer 1 then views amagnified virtual image created by reflective imager 304 throughmagnifying lens 309.

FIGS. 4A-4B illustrate a transflective light modulating displayapparatus using a DRP layer. In general, a transflective displaytypically includes a light modulating layer made up of an array of lightvalves or pixels, each of which represents a pixel of a digital image.Transflective displays can operate in either a reflective mode, in whichthe display primarily relies on ambient light entering from the viewingside for illumination, or in a transmissive or backlit mode, in whichthe display primarily relies on light emanating from an optical cavityopposite the viewing side for illumination. Examples of transflectivelight modulating displays include those which use twisted orsupertwisted nematic liquid crystal layers. Liquid crystal displays ofthese types typically operate by either rotating or leaving unrotatedthe plane of polarization of a beam of linearly polarized light.

The example transflective light modulating display apparatus 400includes an absorbing polarizer 401, a light modulating layer 402,disposed on a side of absorbing polarizer 401, a diffusely reflectingpolarizer 404, disposed on a side of the light modulating layer 402opposite the absorbing polarizer 401, a light trapping layer 405,disposed on a side of the diffusely reflecting polarizer 404 oppositethe light modulating layer 402, and an optical cavity 406 disposed on aside of the light trapping layer 405 opposite the diffusely reflectingpolarizer 404. The example display apparatus 400 further includes anoptional polarization preserving diffuser layer 403.

Operation of the example display 400 will now be described. Withreference to FIG. 4A, a pixel 402 a of a light modulating display 400 isportrayed in the voltage-off state, which produces a bright pixel whenilluminated by ambient light. In this state, unpolarized ambient lightenters the display through absorbing polarizer 401, which passes thatportion of the light having polarization P1, represented by ray 451. Ray451 then passes through light modulating pixel layer 402, where itspolarization is rotated, due to the display being in the voltage-offstate. This produces light 453, which has polarization P2. Light 453 maythen passes through polarization preserving diffuser 403, whichmaintains the polarization P2, and passes on to DRP layer 404, where itis diffused backwards, as ray 455, since DRP layer 404 has been alignedat an orientation which diffusely reflects light having polarization P2.Depolarization of the diffusely reflected ray 455 is typically minimizedto increase the amount of polarization P2. Ray 455 again passes throughdiffuser 403 and then through light modulating pixel layer 402 a, whereits polarization is again rotated to P1, thereby allowing it to passthrough absorbing polarizer and be seen as a bright pixel by viewer 1.

On the right side of FIG. 4A, a pixel 402 b of the light modulatinglayer 402 is portrayed in the voltage-on state, which produces a darkpixel when illuminated by ambient light. In this state, unpolarizedambient light enters the display through absorbing polarizer 401, whichpasses that portion of the light having polarization P1, shown as 451.Ray 451 then passes through light modulating pixel layer 402, where itspolarization remains unchanged, due to pixel 402 b being in thevoltage-on state, thereby producing ray 463, which still haspolarization P1. Ray 463 may then passes through polarization preservingdiffuser 403 and passes on to DRP layer 404, where it is transmitted, asray 465, since DRP layer 404 has been aligned at an orientation so as totransmit light having polarization P1.

Ray 465 then enters light trapping layer 405, which traps the light 465to show a dark pixel to the viewer 1. The light trapping layer 405 maycomprise light absorbing dyes or pigments, or polarization rotatingcomponents. In many cases, some light 465 will be reflected by the lighttrapping layer 405 toward the DRP layer 404. Some light may also passthrough trapping layer 405, be reflected by cavity 406 and re-passthrough trapping layer 405 toward DRP layer 404. In the latter case, theamount of light is typically quite small due to multiple passes throughthe trapping layer 405. With either component of light, the DRP layer404 and underlying trapping layer 405 and cavity 406 may be optimized torotate the polarization of the light to P2 so that it reflects off theDRP layer 404 rather than transmits therethrough.

Referring to FIG. 4B, a pixel 402 a in the voltage-on state (left side),operated in the backlit mode, is portrayed. In the backlit mode, thevoltage-on state produces a bright pixel. Unpolarized light is producedby optical cavity 406, after which it passes, with some possibleattenuation, through light trapping layer 405, to DRP layer 404. DRPlayer 404 diffusely reflects the portion of ray 471 having polarizationP2 back to light trapping layer 405 and thence to optical cavity 406,where it may be reflected, changed in polarization, and recycled backfor another try at getting through DRP layer 404. Any light which issuccessfully recycled in this way contributes the improvement of thelight utilization efficiency of the display system. The portion of ray471 which has polarization P1 is transmitted by DRP layer 404 and passeson through optional diffuser 403 to light modulating pixel layer 402 a.Since pixel 402 a is in the voltage-on state, it passes ray 473 withoutaltering its polarization, to absorbent polarizer 401. Since absorbentpolarizer 401 transmits light having polarization P1, ray 473 leavesdisplay 400 and is seen by viewer 1 as a bright pixel.

To the right side of FIG. 4B, a pixel 402 b in the voltage-off state,operated in the backlit mode, is portrayed. In the backlit mode, avoltage-off state produces a dark pixel. The light produced by opticalcavity 406 reaches pixel 402 b in the same manner as in the voltage-onstate, but in this case, since the pixel 402 b is in the voltage-offstate, the polarization of ray 473 is rotated to polarization P2, afterwhich it is absorbed by absorbent polarizer 401, so that the pixel inthis case as seen as a dark pixel. In this manner, a transflectivedisplay is provided in which the state, e.g. bright or dark state, of apixel changes between backlit mode and ambient mode for a given voltagestate, e.g., on or off. In addition, the diffusely reflecting propertiesof DRP can be used in transflective displays to provide a lighted mattesurface which functions as a light background when the display islighted by ambient light, while providing light recycling when thedisplay is functioning in the backlit mode with low ambient light.

In another embodiment, a contrast enhancing filter using a reflectivepolarizer, such as a diffusely reflective polarizer, is provided. Such acontrast enhancing filter can, for example, provide higher displaybrightness with equal contrast when compared to conventional contrastenhancing filters. Contrast enhancing filters are often used on emissiveinformation displays, many of which are based on an inorganic or organiclight emitting material which is excited in an addressable matrix toform an image. The light emitting material, e.g., phosphor, and backingtypically is a good reflector of visible light. As a result, without acontrast enhancing filter, the reflectivity of the light emittingmaterial can cause the display to “wash-out” under conditions of highambient illumination.

A conventional CE filter shown with an emissive element is illustratedin FIG. 5A. The conventional CE filter 500 typically includes atransparent material, e.g., tinted polarizer 510, with a moderately highabsorptivity for visible light. Light emitted from the emissive element512 passes through the tinted absorber 510 once, whereas ambient lightmust pass through it twice. By way of example, a sheet 510 absorbing 50%of visible light will reduce emitted light by 50% and ambient diffuseand non-diffuse glare by 75%. In practice, the reduction is somewhathigher since the effective reflectivity of the emissive 512 element isusually less than 100% (e.g., due to phosphorescence and wavelengthshifting). In the above example, contrast is increased by 2 fold. Theconventional CE filter 500 may further include an absorbing polarizer514 with or without a tint. The principle of operation is very similarto the simple tinted situation.

While the conventional CE filters using absorbing polarizers improvecontrast, they also substantially reduce brightness. The reduction inbrightness can significantly degrade the appearance of the display.Therefore, a need in the display industry is a contrast enhancing layerthat has a relatively high brightness.

FIG. 5B illustrates an exemplary display which includes a contrastenhancing (CE) filter having a reflecting polarizer in accordance withan embodiment of the invention. The reflecting polarizer may, forexample, be a diffusely reflecting polarizer. The example display 550includes an emissive element 560, such as phosphor, for example, and aCE filter 570, along with optional tinted layer 572. The example CEfilter 570 includes an absorbing polarizer 574 and a reflectingpolarizer 576 typically aligned for highest transmission. The absorbingpolarizer 574 and reflecting polarizer 576 may be provided as alaminate, for example. Typically, the laminate is intimately attached tothe emissive element with the reflective polarizer on the side towardsthe emissive element. The emissive element 560 is typically builtdirectly onto the reflective polarizer 576. This can be accomplished inevacuated displays by suitably degassing the reflective polarizer filmbefore sealing into the display.

One or more optional tinted layer(s) 572 may be disposed either above orbelow the reflective polarizer 576, or both. Providing a tinted layer572 above the reflective polarizer 576, as shown in FIG. 5B, typicallyincreases display contrast with the minimum effect on brightness. Atinted layer 572 disposed below the reflective polarizer 576 will alsotypically increase contrast, with a higher impact on brightness. Theformer position has the advantage of reducing the distance betweenelement 560 and DRP 576.

FIG. 5B shows two light rays interacting with the contrast-enhanceddisplay. One ray is display emitted light ray d. Ray d first interactswith the reflective polarizer 576, splitting the ray into transmittedray d and reflected ray e. Ray d is attenuated by the optional tintlayer 572, and is transmitted by the absorbing polarizer 574. Reflectedray e is scattered by the emissive element 560, producing a reflectedray f. Ray f then follows the same process as ray d. The displaybrightness is then Ray d plus ray f, etc. The ambient ray a is firstabsorbed by the absorbing polarizer 574, leaving about 50% of ray atransmitted. The ray is further attenuated by the tinted absorber 572,and transmitted by the reflective polarizer 576 to the emissive element560. Once reflected from the emissive element 560, the attenuated ray bfollows the same process as ray d.

The reflecting polarizer 576 is typically positioned closer to theemissive element than the absorbing polarizer 574. This can improve theimage by reducing the distance between the reflecting polarizer and theemissive element. Since the emissive element will typically emit lightover a wide range of angles, the image could appear diffused if thedistance between the reflective polarizer 576 is significantly largerthan the lateral dimensions of the emissive element 560. In oneembodiment, the distance between the reflecting polarizer 576 and theemissive element is 3 times the lateral distance of the emissive elementor less. In another embodiment, the distance between the reflectingpolarizer 576 and the emissive element is 2 times the lateral distanceof the emissive element or less. In a further embodiment this distanceis equal to or less than the lateral distance of the emissive element.

The color of the tint absorber, if any, and the absorbing polarizer, iftinted, can be optimized with displays that produce colored light. If,for example, a display produces green light, the tint will ideally havea relatively high absorption in the red and blue, and a relatively lowabsorption in the green. This concept may be used in full color displaysby providing a matrix of the color of the tinted sheet to match thecolor of the individual emissive elements.

An emissive element may, for example, be a pixel or a subpixel elementwhich emits a particular color. The CE filter using a reflectivepolarizer and an absorbing polarizer may be used in a number ofdifferent systems having emissive elements. Such systems include, by wayof example, fluorescent illuminated displays, electroluminescentdisplays, organic and inorganic light emitting diode displays, vacuumfluorescent displays, field emissive displays (FED), and plasmadisplays.

Additionally, while diffusely reflecting polarizers work particularlywell, the invention is not so limited. Other types of reflectivepolarizers which provide linearly polarized light may be used. Theseother reflective polarizers include, for example, the multilayerreflective polarizer and the cholesteric reflective polarizer discussedabove.

Using a CE filter with a reflecting polarizer (e.g., a DRP), a higherdisplay brightness can be achieved for a given contrast than withconventional CE filters without reflecting polarizers. FIGS. 5C-5Eillustrate various characteristics of the CE filter having a reflectivepolarizer (hereinafter RP CE filter) of FIG. 5B and the conventional CEfilter of FIG. 5A. FIG. 5C is a graph illustrating the relativebrightness of ambient diffuse and specular glare vs. emitted brightnessas a function of tint. Curves 582 and 586 show the relative brightnessof emitted and ambient light, respectively, using the RP CE filter.Curves 584 and 588 show the relative brightness of emitted and ambientlight, respectively, using the conventional CE filter. As can be seenthe RP CE filter produces higher brightness at all tint levels.

FIG. 5D is graph illustrating the contrast ratio (i.e., the ratio ofdisplay brightness to ambient brightness) as a function of tint for theRP CE filter (curve 590) and the conventional filter (curve 592). Togenerate the graph, ambient light and display light were eacharbitrarily set to 1 with the contrast ratio changing with changes inthe ambient light. As can be seen from the graph, the highest contrastwill be achieved at the highest level of tint. In addition, the curves590 and 592 substantially overlap, thus showing that RP CE filterprovides higher display brightness for the same contrast ratio.

FIG. 5E is a graph illustrating another contrast characteristic, ambientbrightness less emission brightness, as a function of single pass tinttransmission for the two CE filters. This contrast ratio may be used toprovide an indication of the legibility (e.g., the speed and straininvolved in reading information) of a display. Curve 594 illustratesthis characteristic for the RP CE filter and curve 596 illustrates thecharacteristic of the conventional CE filter. As can be seen, the RP CEfilter provides higher contrast than the conventional CE filter at alllevels.

Diffusely reflecting polarizers of the present invention can also beused to alter and improve the appearance and viewability of existingbacklit light valve or modulating displays, such as backlit twistednematic and supertwisted nematic displays. FIG. 6 illustrates a backlitdisplay apparatus 600 which includes a light modulating layer 606, alight cavity 610 for providing light for illuminating the lightmodulating layer 606, and a DRP layer 603 disposed on a viewing side ofthe light modulating layer 606. The display apparatus 600 may, forexample, be an LCD with a liquid crystal light modulating layer made upof an array of liquid crystal elements representing pixels of a digitalimage. The light cavity 610 may, for example, include a light source andusually a light guide, for example.

Light rays 651 and 653 from the illuminator 610 are typicallyunpolarized, but may have some degree of prepolarization, and displayapparatus 600 is typically provided with absorbing polarizers 605 and604. Light modulating layer 606 then either transmits or blocks ray 651in an imagewise manner, depending on whether voltage is applied or notapplied to individual pixel elements. It has been found that interposingdiffusely reflecting polarizer layer 603 between display 620 and viewer1 alters the appearance of display 620 in ways which make it appearunique in comparison to other displays. It is also possible toincorporate a suitable amount of directionally varying transmissiondiffusivity into layer 603 to adjust the angles at which the display canbe viewed, without decreasing the brightness of the display to anunacceptable level. This can be accomplished by, for example,controlling the amount of the dispersed phase, as described above.

Improved viewability of information displays is not limited toelectronic displays. In a broader sense, printed matter and graphicalportrayals such as signs and billboards are also information displays.Even more generally, it known that the viewing of many objects,especially those requiring detailed viewing, can be aided by use ofpolarized light, since light polarized in the plane of incidence oflight striking the object, called p-polarized light, is likely to have alower reflectivity, and hence a lower tendency to produce glare, thanthe s-polarized light, which is the light polarized perpendicular to theplane of incidence relative to the surface of the object being viewed.However, because of the losses of light inherent in producing polarizedlight, it is sometimes difficult to produce polarized light in aneconomically feasible manner in the quantities needed for viewing largedisplays such as signs and billboards, or for general room lighting. Ithas been found, however, that diffusely reflecting polarizers can beused to produce light which retains many of the benefits of polarizedlight while still utilizing substantially all of the original,unpolarized, light.

FIG. 7A-B illustrate a lighting apparatus in accordance with anotherembodiment. The exemplary lighting apparatus 700 may, for example, be anoverhead lighting system used in a room, such as an office. The examplelighting apparatus 700 includes a light source 707 and a reflector 708.The light source 707 typically provides unpolarized light to thereflector 708. The light source 707 may include a reflective backing andmay be arranged to direct lighting substantially only in directionstoward the reflector 708. The illustrated reflector 708 specularlyreflects light 705 of a first polarization (P1) to provide P1 light witha first distribution and diffusely reflects light 706 of a second,different polarization (P2) to provide P2 light with a seconddistribution different than the first distribution. The differentpolarizations and distributions may be used in different mannersdepending on the application.

In the example embodiment, as best illustrated in FIG. 7B, the reflector708 includes a diffusely reflecting polarizer 701 disposed closer to thelight source 707 and a specular reflector 702 disposed further from thelight source 707. The two components 701, 702 of the reflector 708 may,for example, be fixedly attached (e.g., by lamination). The diffuselyreflecting polarizer is arranged to diffusely reflect P2 light 706 andspecularly transmit P1 light 705.

In use, the light source 707 illuminates the DRP reflector 708 withlight, e.g., incident ray 750. A component 703 of the light having afirst polarization P2 is transmitted by the diffusely reflectingpolarizer 701, specularly reflected by the specular reflector 702, andspecularly retransmitted through the diffusely reflecting polarizer 701to provide specularly reflected P1 light 705. A component 704 of thelight having a second, different polarization P2 is diffusely reflectedby the diffusely reflecting polarizer 701 to provide diffusely reflectedlight 206 having a second distribution different than the firstdistribution.

The reflector 708 may be disposed to curve about the light source 707such that the specularly reflected P2 light 705 has a substantiallyuniform distribution and diffusely reflected P1 light has a scattereddistribution. The reflector 708 may disposed around the light source 707so that it focuses or directs light in to a concentrated area. In thiscase, P1 light 705 can be used to provide concentrated polarizedlighting of one polarization, while diffusely reflected P2 light 706 canbe used to provide general room lighting having an orthogonalpolarization. In this manner, both polarizations of light from source707 are utilized, while the benefits of polarization for lighting for aparticular task requiring higher light intensity in a smaller area areretained.

In one particular embodiment, the diffusely reflected light of the firstpolarization is p-polarized light (vertically-polarized light) and thespecularly reflected light of the second, different polarization iss-polarized light (horizontally-polarized light). When used as a ceilinglight, the reflector 708 may be arranged to direct the s-polarized lighttoward the floor. In this manner, the s-polarized light whichcontributes to glare is directed to a location which minimized glare. Onthe other hand, the p-polarized light is scattered to provide generalambient light for the room.

The lighting apparatus may further include reflective louvers 709arranged to specularly reflect P1 light and diffusely reflect P2 light.Each reflective louver 709 may include, on one or both sides, a DRPlayer arranged to diffusely reflect P1 light and specular transmit P2and a specular reflector disposed behind the DRP layer and arranged tospecularly reflect P2 light, similar to reflector 708. The louvers 709typically are pivotally mounted to the lighting apparatus frame to allowthe direction of reflected light, and in particular the specularlyreflected light, to be controlled.

Where the louvers 709 (and reflector 708 as noted above) are configuredto specularly reflect s-polarized light and diffusely reflectp-polarized light, the louvers 709 may be pivoted so that thes-polarized light, which causes glare, is specularly reflected in adirection which minimizes glare, e.g., toward the floor. The p-polarizedlight, which contributes little to glare, is advantageously diffuselytransmitted to provide, e.g., general room lighting.

The louvers 709 are typically positioned to receive light from the lightsource 707 via reflector 708. In the example embodiment, the light isreceived via a reflector 708 having a DRP layer for diffusely reflectingP2 light (e.g., p-polarized light) and a specular reflector forspecularly reflecting P1 light (e.g., s-polarized light). In alternateembodiments, a reflector 708 which simply specularly reflects may beused with the louvers 709 functioning as the only diffusely reflectingand specularly reflecting element.

While a DRP/specular reflector louvers 709 and reflectors 708 areillustrated and discussed above, the invention is not so limited. Othercombinations of specular and diffusely reflecting material may be usedfor the reflector 708 and/or louvers 709 and are intended to fall withinthe scope of the invention.

In one alternate embodiment, either or both of the components (i.e., thereflector 708 and/or louvers 709) includes a multilayer reflecting filmdisposed closer to the light source and a diffusely reflecting surface,disposed further from the light source. In use, the multilayerreflecting film specularly reflects P1 light (e.g., s-polarized light)and transmits P2 light (e.g., p-polarized light). The diffuselyreflecting surface diffusely reflects P2 light, which is retransmittedby the multilayer reflecting film to provide diffusely reflected P2light. The diffusely reflecting surface may, for example, be a texturedmetal surface.

In another embodiment, either or both of the components includes amultilayer reflective film, having one or two structured surfaces fordiffusely reflecting P2 light (e.g., p-polarized light), and a specularreflector, disposed further from the light source, for specularreflecting P1 light (e.g., s-polarized light). The P1 light is thenretransmitted by the multilayer film to provide specularly reflected P1light.

FIGS. 8A-8D illustrate security labels using one or more DRP layers inaccordance with further embodiments of the invention. Referring now toFIG. 8A, example security label 890 comprises label portion 800 andsecurity portion 810. Label portion 800 comprises information portion804, which might be, for example, a paper or film substrate whichcarries printed, graphic, or other information. Information layer 804may be attached, by means of adhesive layer 805, to protective layer 806for example. Protective layer 806 may be permanently attached toinformation layer 804, e.g., where the adhesive layer 805 is a permanentadhesive. Alternatively, adhesive layer 805 can be a pressure-sensitiveadhesive, and layer 806 can be a removable adhesive liner which protectslayer 805 until the label 890 is ready to be attached to a package orother substrate. In the event that protective layer 806 is a permanentprotective layer, additional adhesive layer 807 can be provided toenable the label to be attached to the substrate to be labeled.

Security portion 810 of label 890 comprises a first diffusely reflectingpolarizing layer 801 a, aligned in a first direction, and a seconddiffusely reflecting polarizing layer 801 b, aligned in an orthogonaldirection, so as to form a pair of crossed diffusely reflectivepolarizers. In addition, protective layer 803 can be provided.

In use, the authenticity of label 890 can be determined by viewing thelabel from a first position 1, which produces a hazy view of theinformation carried on information layer 804, and then viewing the labelat a grazing angle, as indicated by second position 3, in which case theinformation becomes more clear. Authentication may further be aided byincorporating into information layer 804 some small or intricateprinting or other graphics which would be sensitive to the hazeexperienced when viewing an authentic label from position 1.

Referring to FIG. 8B, an alternative embodiment results from insertingclear spacer layer 809 between information layer 804 and first diffuselyreflecting polarizer layer 801 a. Preferably, layer 809 is air or otherfluid material which can be excluded from between polarizing layer 801 aand information layer 804 by the application of pressure to protectivelayer 803. Prior to application of pressure, viewing of informationlayer 804 from either position 1 or position 3 is blocked by securityportion 820, which includes clear layer 809. With pressure applied, sothat layer 809 is eliminated, the situation depicted in FIG. 8A occurs,wherein information layer 804 of an authentic label is hazy, thoughvisible, from position 1, and clearly visible from position 3. Suitablepressure can be applied by a transparent plate, or by a suitable ring orother device containing an aperture through which information layer 804can be viewed during the application of pressure. Suitable fluidmaterials for layer 809 include air or other gases, as well as clearliquids. Layers 804 and 801 a can be held apart when pressure is notapplied by mechanical means, such as by incorporating a slight wavinessinto these layers, or by fluid pressure, if layer 809 is made of a fluidwhich can be pressurized. It will be appreciated that providing asuitable vent or reservoir for fluid excluded from layer 809 is alsouseful.

Security labels using DRPs can also be made which employ a separateviewing device, which is used with the label by the personauthenticating it. Referring to FIG. 8C, viewing layer 802 can be anytype of polarizing layer, such as an absorbent polarizer or otherpolarizing material. The distance d between viewing layer 802 andprotective layer 803 is not critical, and can be any convenient distancewhich allows viewer 1 to read information layer 804. Since viewing layer802 is reusable, it is not subject to the same cost constraints as adisposable layer provided with the label would be. Security layer 830comprises diffusely reflecting polarizer layer 801 and protective layer803. Viewer 1 views information layer 804 through viewing layer 802 andfirst views information layer 804 in, for example, an orientation inwhich the transmission axis of viewing layer 802 is orthogonal to thetransmission axis of layer 801. Viewed in this manner, information layer804 of an authentic label appears hazy. Viewing layer 802 is thenrotated to an orientation in which its transmission axis is parallel tothe transmission axis of layer 801, whereupon information layer 804 ofan authentic label appears clear.

In yet another embodiment, portrayed in FIG. 8D, security portion 840comprises clear spacer layer 809, diffusely reflecting polarizer layer801, and protective layer 803. Viewer 1 then views information layer 804through viewing layer 802, which is oriented, for example, with itstransmission axis orthogonal to the transmission axis of layer 801. Inthis case the view of information layer 804 is completely blocked,rather than being merely hazy, as it was when spacer layer 809 wasabsent, as was the case in the previous example, portrayed in FIG. 8C.Viewing layer 802 is then rotated to an orientation in which itstransmission axis is parallel to the transmission axis of layer 801,whereupon the information layer 804 appears clear, as it did in theprevious example.

It will be appreciated that protective layer 803 is optional in theabove embodiments, and that in some applications of the invention, layer801 may be sufficiently durable to be used without additionalprotection. Other variations on the above embodiments will be readilyapparent to those of ordinary skill in the art.

Electroluminescent panels are a convenient and efficient source of lightfor many applications, due to their compactness and light weight. Manysuch applications require, in addition, that the light panel producepolarized light. One example of this requirement is the liquid crystaldisplay. There is therefore a need to efficiently produce polarized rayusing such sources, without adding excessive weight or size to thesystem.

Referring to FIG. 9, an electroluminescent panel 900 using a diffuselyreflecting polarizing layer 901 for providing polarized light isprovided. In operation, a light emitting layer 903 (e.g. phosphor layer)emits light when a voltage is applied between transparent electrodes 902and 904 by means, for example, of power source 910. Layer 903 emitslight in all directions, so reflector 905 may be added to direct lightin the general direction of DRP layer 901. It will be appreciated thatelectrode 904 could be a metallic reflective electrode, therebyobviating the need for separate reflector 905. Light emitted by emissivelayer 903 is transmitted through transparent electrode 902 to diffuselyreflecting polarizing layer 901, which transmits light 906, havingpolarization P1, and which diffusely reflects light 907 havingpolarization P2. Light 907 is further diffused and reflected, withchanges in polarity, with portions of it eventually emerging from layer903 as light 906′, which also has polarity P1. A portion 907′ is againdiffusely reflected back into layer 903, where it again undergoeschanges in polarization, due to diffusion, reflection, and otherpolarization-altering phenomena, until it eventually emerges from layer903 and is transmitted as light 906″. As a result of this recycling oflight, the amount of properly polarized light emitted byelectroluminescent panel 900 is increased. It may be useful, in somecases, to incorporate into layer 903, in addition to the emissivephosphor material, materials to increase the reflection andpolarization-altering effects of emissive layer 903.

The DRP layer 901 and emissive layer/reflector may further be optimizedto recycle light. For example, the DRP layer 901 may, for example,substantially depolarize diffusely reflected light to facilitate lightrecycling with an emissive layer having less depolarizationcharacteristics. Alternatively, the angular depolarizationcharacteristics of the two components may be set such that the emissivelayer significantly depolarizes light at incident angles containingrelatively large amounts of non-depolarized light and vice versa, asdiscussed above.

As noted above, the present invention is applicable to a number ofdifferent devices using diffusely reflecting polarizers. Accordingly,the present invention should not be considered limited to the particularexamples described above, but rather should be understood to cover allaspects of the invention as fairly set out in the attached claims.Various modifications, equivalent processes, as well as numerousstructures to which the present invention may be applicable will bereadily apparent to those of skill in the art upon review of the presentspecification. The claims are intended to cover such modifications anddevices.

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 21. An optical device comprising an emissive element and adiffusely reflecting polarizer disposed to receive light therefrom, saiddiffusely reflecting polarizer attached to the emissive element.
 22. Theoptical device of claim 21, further comprising an absorbing polarizer.23. The optical device of claim 22, wherein the absorbing polarizer isaligned with the diffusely reflecting polarizer.
 24. The optical deviceof claim 22, wherein the absorbing polarizer is laminated to thediffusely reflecting polarizer.
 25. The optical device of claim 21,further comprising a tinted layer.
 26. The optical device of claim 21,wherein the diffusely reflecting polarizer is built directly onto theemissive element.
 27. The optical device of claim 21, wherein theemissive element comprises phosphor.